Optical spectra of ${\mathrm{Pr}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ crystals: Evolution of in-gap states with electron doping

The systematic change of optical conductivity spectra with carrier doping and their anisotropy have been investigated for single crystals of ${\mathrm{Pr}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$; the crystals were grown by a traveling solvent floating-zone method. The evolution of in-gap states with x has shown up not only in the growth of midinfrared absorption below the charge-transfer gap but also in higher-lying interband transitions between O 2p and Cu 4s states. Possible roles of the reducing procedure, which is indispensable for producing superconductivity in the electron-doped cuprates, are also discussed in terms of observed changes of the optical spectra.